US2027949A - Image suppression arrangement - Google Patents

Description

Jan. 14, 1936.

J. YpLLEs IMAGE SUPPRESS ION ARRANGEMENT Filed Oct. 28, i933 SIG/V44 Eli 576) INVENTOR JACOB YOLLES ATTORNEY Patented Jan. 14, 1936 UNITED STATES PATENT OFFICE Jacob Yolles, Brooklyn, N. Y.,. assignor to Radio Corporation of America, a corporation of Delaware Application October 28, i933, Serial No. 695,596

4 Claims.

My present invention relates to superheterodyne receivers, and more particularly to an improved arrangement for obtaining image suppression in a superheterodyne receiver.

It has been proposed in the past to deal with the problem of image suppression in a superheterodyne receiver of the broadcast type by tapping the input coil of the first radio frequency selector circuit, and connecting the tap on the input coil tothe grid of the first tube. Such a device has proven to be of considerable value in minimizing to a great extent the most troublesome of the image frequencies which occur when operating a superheterodyne receiver in the broadcast frequency range. image suppression function was found to practically disappear when it was attempted to embody the tapped input coil construction in an arrangement wherein the selector circuit is coupled to the signal collector by a combined magnetic and capacitive coupling having a flat gain characteristic over the broadcast frequency range.

Accordingly, it may be stated that it is one of the main objects of the present invention to provide in a superheterodyne receiver a tapped input coil in the signal selector for obtaining image suppression, and combined capacity and magnetic coupling to the tapped coil so as to obtain both a flat gain characteristic and undisturbed image suppression.

Another important object of the invention is to provide between the antenna circuit of the superheterodyne receiver and the input electrodes of the first tube a tunable coupling network operative for the broadcast frequency range, which network is particularly characterized by the fact that it includes a connection from the grid of the first tube to an intermemediate tap on the antenna transformer secondary coil, and combined magnetic andcapacity coupling from the transformer primary to the portion of the tapped coil "between the intermediate tap and the low A. C. potential terminal of the taped coil.

Still other objects of the invention are to improve generally the efiiciency of superheterodyne receivers employing devices for securing a flat selector circuit gain characteristic and image suppression, and to particularly provide such a receiver which is not only reliable in operation throughout the broadcast frequency range, but economically assembled in a radio receiver.

The novel features which I believe to be characteristic of -my invention are set forth in par- However, the

Fig. 1 diagrammatically shows a portion of a superheterodyne receiver embodying the present invention.

Fig. 2 is an analysis of the functioning of the invention.

Fig. 2a is a modified form of the invention.

Referring now to the accompanying drawing wherein like reference characters in the two figures designate similar circuit elements, a superheterodyne receiver of purely conventional design is shown in Fig. 1. The signal collector, or absorption, circuit includes the customary grounded antenna I. The first tube 2 is represented as of the screen grid type, but any other type of tube may be employed in the tunable radio frequency amplifier following the signal collector. The coupling network between the antenna l and the input electrodes of tube 2 includes a transformer consisting of the high impedance primary coil 3 and the secondary coil. The latter consists of two portions 4 and 5, the primary coil 3 being magnetically coupled with secondary portion 4 as shown at M. The numerals and 5 may designate the part of a single winding, or a pair of individual, separated coils.

The signal grid of the tube 2 is connected by lead I to an intermediate point 6 on the secondary coil, the point 6 being chosen in a predetermined mannerdepending on local image interference conditions and the intermediate frequency employed. Coil 3 has little, or no, magnetic coupling with the secondary portion 5, and the variable tuning condenser 8 is shunted across the entire secondary coil. Capacity coupling is provided between coil 3 and the following tunable input circuit by connecting one end of the coil 9 to the ungrounded side of coil 3, and disposing the coil 9 adjacent secondary coil portion 4 close to the tapping point 6. The dotted line representation C designates the inherent capacity between the coil 9 and the secondary portion 4. This disposition of the coil 9 is an important feature of the invention, image interference suppression having been found possible in an arrangement of the type shown in Fig. 1 only by utilizing this disposition. Coil 9 functions as a condenser, and is so employed as an economical expedient.

In actual construction the coils 9, 3 and secondary portions 4, 5 may be disposed on a common cylindrical form. The coil 9 may comprise if desired two turns of insulated wire wound on the secondary adjacent the tap point 6, the primary coil 3 being spaced from the secondary but being wound on the form so as to have magnetic coupling with the portion 4. One terminal of coil 9 is left free, as shown in Fig. 1.

The anode circuit of tube 2 is coupled, as at H], to the tunable input circuit of the first detector II. The variable tuning condenser 12 of the latter has its rotor arranged for mechanical unicontrol with the rotor of tuning condenser 8. The local oscillator I3 is provided with a tuning condenser 14 whose rotor is arranged for unicontrol with the rotors of condensers 8 and [2.

The dotted line I5 denotes the common control shaft of tuning condensers 8, l2 and [4. As is well known to those skilled in the art, the amplifier and first detector are tuned through a range of 550 to- 1500 kilocycles (the broadcast range), while the local oscillator is tuned through a frequency range which differs at all times from the desired signal range by the operating intermediate frequency.

The actual construction of the networks I l and I 3 is so well known to those skilled in the art that a detailed circuit description is believed to be unnecessary. Furthermore, it will be understood that in the local oscillator network there is usually associated with the tuning condenser l4 padding, or trimmer, condensers for insuring constancy of the operating intermediate frequency throughout the tuning range of the receiver. Such auxiliary condensers are described by W. L. Carlson in Patent #1,740,331 of December 17th,

1929. The intermediate frequency output of the network H may be impressed upon any well known type of intermediate frequency amplifier, then demodulated and utilized in an audio amplifier, and/or reproducer such as a loud speaker.

For reasons which are well known the operating frequency range of the local oscillator is usually chosen to be higher than the desired signal frequency range. Thus, if it has been decided that an intermediate frequency of 1'75 kilocycles, as a given example, is to be employed, then the operating range of the local oscillator would be 725 kilocycles to 1675 kilocycles. It will, therefore, be seen that at any frequency in the desired signal frequency range, the image frequency that can be received will be equal to the desired signal frequency plus twice the intermediate frequency value. Hence, the undesired image frequency is always disposed above the desired carrier frequency.

For this reason, it has heretofore been proposed that the signal grid of the first tube of the receiver be connected to a point intermediate the antenna transformer secondary. Such a tapped secondary circuit results in a dissymmetry such that the radio frequencies above the desired signal carrier are discriminated against in favor of the radio frequencies lying below the-desired carrier. Of course, the effectiveness of suppression of the undesired image frequencies varies with the tuning of the signal selector circuit. It is for this reason that the location of the tapped point 6 depends upon several factors, the most important of which are the image interference conditions in the locality where the receiver is operated. Actual experience has demonstrated that the tapping of the transformer secondary, regardless of the theory of the action, results in a considerable diminution of image interference.

Now, when such a tapped input coil arrangement as has been described above as being known in the prior art, is attempted to be utilized in a coupling network of the type shown in Fig. 1, it is found that the desirable use of combined magnetic and capacitative coupling to obtain flat gain vitiates the feature of improved image suppression. In other words, when it is desired to incorporate the tapped input coil arrangement in a receiver employing combined magnetic and capacitive coupling in aiding phase, it is found that image suppression effects are not secured. A consideration of Fig. 2 will demonstrate the reason for such failure to secure the desired result, as well as to show how the present invention solves the problem presented.

In Fig. 2 the tube 2 is shown as having its input electrodes coupled to a general source of signal energy. This signal energy source may be the antenna circuit I of Fig. 1, or it may even be the plate circuit of a preceding amplifier. It is also to be understood that the tube 2 may be the first detector tube itself. The high impedance coil 3 is coupled to the secondary of the transformer, but the secondary portions 4 and 5 have been redrawn so as to show the tuning condenser 8 and the coil 5 arranged in series resonance in shunt with the secondary portion 4. It will be realizedthat this rearrangement of the circuit elements in no way changes the electrical relations between them. The coil 9 provides the capacitative coupling C between the primary 3 and the secondary.

The conventional and well known practice is to dispose the capacity coupling coil 9 adjacent the high alternating current potential point of the coil portion 5. That is to say, in actual practics, and without regard to image suppression, the coil 9 in Fig. 1 would be disposed adjacent the upper terminal of the secondary portion 5. Such a disposition is equivalent to providing the capacity coupling C between the junction of the secondary portion 5 and condenser 8 of the high alternating current potential terminal of the primary coil 3. If now the secondary be tapped at point 6, it is easy to show that image suppression will not take place as long as the coil 9 is disposed adjacent the condenser side of the secondary portion 5. The elements 5 and 8 are in series and across the tube input electrodes, and, therefore, form a series resonant path between the gridand cathode of tube 2. Although not always tuned to the image frequency, the series path does attenuate the higher radio frequencies and greatly improves the resistance of the selector to image frequencies as effective between the input electrodes of the tube 2. To secure the image suppression function it is required for the effectiveness of the series circuit that no voltage be introduced in the series path at the common junction of elements 5 and 8. The reason for this may be summed up as follows:

The operation of the elements 5 and 8 in series resonance whereby the impedance across the tube input is greatly reduced for image interference calls for the source of signal to be across these elements since only then is the current through the elements 5 and 8 equal, the condition for equal and opposite voltages across 5 and 8 and consequently a minimum across the tube. If signal energy is applied as shown by the dotted line, there is an unbalance of the currents through 5 and 8, and the difference in voltage across these elements is no longer a minimum, and increases as the capacity coupling is raised, becoming of such proportion when adequate capacity coupling is employed that it vitiates the effect of image suppression. After discovering that the introduction of exciting energy into the secondary portion was responsible for the failure to secure image suppression, actual construction demonstrated that introduction of the exciting energy, both inductive and capacitative, into the secondary portion 4 yielded the desired image suppression function, and in no way detracted from the flat gain characteristic of the combined coupling.

It will, therefore, be seen that the present invention employs a tapped input coil for obtaining image suppression, and combined magnetic and capacitative coupling between the tapped input coil and the source of signal energy, the combined couplings being so related as to obtain both fiat gain and undisturbed image suppression. It is also a desirable feature of the present invention to employ as the primary of the tapped secondary coil a high impedance coil; that is, one whose inductance is so high that its natural period is below but close to the lowest frequency of the broadcast range. Such a high impedance primary, in a tapped coil image suppression arrangement, is therefore shown feeding the exciting energy into the lower portion of the tapped secondary.

In Fig. 2a is shown the invention applied to a combined magnetic and capacitative coupling 3, 9, 5, 4 used between tubes 2 and 2. The primary coil 3 is arranged, as shown in Fig. 2a, with respect to the coil Sand portions 4 and 5. The action involves the previous explanation.

While I have indicated and described two systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. An image suppression circuit for radio receivers of the superheterodyne type comprising in combination, a tunable signal selector circuit having an inductance coil shunted by a variable tuning condenser, an electron discharge tube coupled to the selector circuit so that the signal grid of the tube is effectively connected to a tap on an intermediate point of the inductance coil whereby the coil is divided into two portions, a-

source of signal energy, means for transferring the signal energy from said source to the selector circuit comprising a high impedance primary coil for magnetically coupling the source to the portion of the tapped coil which is between the tapped point thereof and the low alternating'current potential end of the coil and a capacitive coupling means between the signal source and the inductance coil, said magnetic and capacitive coupling being arranged so that substantially no signal voltage is introduced in the other portion of the tapped coil.

, 2. In an image suppression circuit of the type wherein a tapped input inductance coil of a radio frequency selector circuit is employed, said tap connecting to a grid of an amplifier tube and wherein there is provided combined magnetic and capacitive coupling between the tapped coil and a source of signal energy, said magnetic coupling being disposed with relation to the tap on the input inductance coil so that signal voltage is introduced substantially only in the portion of the input inductance coil which is between the tapped point and the low alternating current potential end thereof, said capacitive coupling being disposed adjacent the input inductance coil close to the tapping point thereof.

3. In an image suppression circuit of the type wherein a tapped input inductance coil of a radio frequency selector circuit is employed, said tap connecting to a grid of an amplifier tube, and wherein there is provided combined magnetic and capacitive coupling between the tapped coil and a source of signal energy, said combined coupling acting normally to practically suppress the image suppression function of the circuit, the improvement which comprises disposing the magnetic coupling between the source of signal energy and the input inductance coil with relation to the tap on the input inductance coil so that signal voltage is introduced substantially only in that portion of the input inductance coil which is between the tapped point and the low alternating current potential end thereof and disposing the capacitive coupling between the signal circuit and the input inductance coil close to the tapping point of the input inductance coil whereby the .image suppression characteristics of the circuit are maintained despite the combined magnetic and capacitive coupling between the input coil and the source of signal energy.

4. An image suppression circuit for radio receivers of the superheterodyne type comprising in combination, a tunable signal selector circuit including the secondary of a transformer said secondary acting as an inductance coil, a variable tuning condenser shunted across the secondary and arranged to tune the selector circuit over a range of frequencies, an electron discharge tube coupled to the selector circuit so that the signal grid of the tube is effectively connected to a tap on an intermediate point of the secondary whereby said secondary winding is divided into two portions, a source of signal energy, means for transferring the signal energy from said source to the selector circuit comprising the primary winding of the transformer, said primary winding having a high impedance value, the inductance of said primary winding being so high that its natural period is below but close to the lowest frequency of said range of frequencies, said transformer acting to magnetically couple the source to the portion of the tapped coil which is between the tapped point thereof and the low alternating current potential end of the coil and means for capacitively coupling the signal source to the inductance coil said means comprising a coil having one end thereof connected to one end of the primarywinding of said transformer and the other end free, said coil being disposed adjacent the secondary winding close to the tapping point thereof, said mag- 55

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